Third eyelid

The Nictitating membrane is a transparent or translucent third eyelid present in some animals that can be drawn across the eye for protection and to moisten the eye while also keeping visibility. Various reptiles, birds and sharks have a full nictitating membrane while in many mammals there is a small vestigial remnant of the membrane present in the corner of the eye. Some mammals such as polar bears, seals and ardvarks also have a full nictitating membrane. It is often called a third eyelid or haw and may be referred to as the plica semilunaris or palpebra tertia in more advanced applications.
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Third eyelid see membrana nictitans, nictitating membrane. third eyelid eversion the free margin of the third eyelid is rolled outwards because of a [more]Third eyelid - Community and Resources

Noun
1. a protective fold of skin in the eyes of reptiles and birds and some mammals
(synonym) nictitating membrane
(hypernym) protective fold
(hyponym) haw
(part-holonym) eye, oculus, optic

See Nictitating membrane.